Stable PbS colloidal quantum dot inks enable blade-coating infrared solar cells

Infrared solar cells are more effective than normal bandgap solar cells at reducing the spectral loss in the near-infrared region, thus also at broadening the absorption spectra and improving power conversion efficiency. PbS colloidal quantum dots (QDs) with tunable bandgap are ideal infrared photovoltaic materials. However, QD solar cell production suffers from small-area-based spin-coating fabrication methods and unstable QD ink. Herein, the QD ink stability mechanism was fully investigated according to Lewis acid–base theory and colloid stability theory. We further studied a mixed solvent system using dimethylformamide and butylamine, compatible with the scalable manufacture of method-blade coating. Based on the ink system, 100 cm2 of uniform and dense near-infrared PbS QDs (~ 0.96 eV) film was successfully prepared by blade coating. The average efficiencies of above absorber-based devices reached 11.14% under AM1.5G illumination, and the 800 nm-filtered efficiency achieved 4.28%. Both were the top values among blade coating method based devices. The newly developed ink showed excellent stability, and the device performance based on the ink stored for 7 h was similar to that of fresh ink. The matched solvent system for stable PbS QD ink represents a crucial step toward large area blade coating photoelectric devices. Graphical Abstract Supplementary Information The online version contains supplementary material available at 10.1007/s12200-023-00085-0.


DLVO theory
According to the DLVO theory, there are two independent forces between particles in the liquid.The first is the van der Waals gravitation, which causes the particles to coagulate.The second is the electrostatic repulsion caused by the overlap of the diffused electrostatic double layer, which is the reason for maintaining colloid stability.The stability of colloids depends on the larger one.
Firstly, according to Hamaker's hypothesis, the gravitational potential energy resulting from van der Waals attraction between colloidal particles is expressed as where r is the particle radius in m, H is the shortest distance from the particle surface in m, and A is the Hamaker constant in J.
Secondly, colloidal particles have an electric double-layer structure.Once the ligand on the QDs surface is dissociated in a strong Lewis basicity environment, most of the PbI3 ions will attach to the charged surface of the QDs, forming a "stern" layer, which together with the QDs forms colloidal particles; Most PbI + ions will form a diffusion layer around colloidal particles due to electrostatic interaction.The electric double-layer is composed of colloidal particles and their diffusion layer is electrically neutral, so the diffusion layer acts as a shielding layer to shield charged particles from electrostatic repulsion.However, when colloidal particles are close to each other, the diffusion layer will overlap.Overlapping will destroy the charge distribution of the diffusion layer and will affect the electrostatic balance and potential of the electric double-layer, thus damaging the shielding of colloidal particles and generating electrostatic repulsion between each other.The expression of electrostatic repulsion potential energy is where r is the radius of colloidal particles in m, H is the shortest distance between particle surfaces in m, N0 is the number density of positive or negative ions in the solution, K is the Boltzmann constant, and T is temperature.The definition formula of where  0 is the surface potential of colloidal particles,  is the ionic valence state, and e is the electric quantity of the electron.
In Eq. ( 2),  −1 is the thickness of the electric double-layer, and its definition formula is where  is the dielectric constant of the solvent.
According to Eqs. ( 2), (3), and ( 4), the electrostatic repulsive potential energy is not only related to the geometric factors r and H between colloidal particles but also depends on the surface potential, electrolyte concentration, ionic valence state, and solvent dielectric constant.
The total potential energy is the sum of gravitational potential energy and repulsive potential energy.

Figure S1 .
Figure S1.a) Photos of quantum dots dispersed in different solvents.b) The absorption spectra of quantum dots dispersed in a mixed solvent system after 4 hours of storage.

Figure S2 .
Figure S2.Photos of the bottom of QD ink after 4 hours of storage.

Figure S3 .
Figure S3.a) XRD diagram of quantum dot film b) EQE and integral current curves of two devices (a) (b)

Table S1 .
Chemical parameters of different solvents

Table S2 .
Physical parameters corresponding to the absorption curves of Figure2c

Table S3 .
Summary of previously reported QDs IRSC and our work.N/A refers to non-reported values.